1. Field of the Invention
Embodiments of the invention provide methods for creation of a solid state structure which emits light when current is passing through. The solid state structure is fabricated using solution-based processes. Fabricated structures are based on the polymer light emitting devices (PLEDs). An example of a PLED is shown in FIG. 1.
2. Description of the Related Art
“Electroluminescence” is light emitted by a solid through which an electric current is passing. This light is in addition to the normal thermal emission of the solid. Electroluminescence implies the presence of electronic excited states with populations larger than their thermal equilibrium value.
One electroluminescent device is an organic light emitting device, or OLED. Successful operation of an OLED has several prerequisites. These may be best illustrated by drawing a parallel with a common gas discharge bulb. The gas discharge bulb and the OLED have two contacts—anode and cathode—for applying an external voltage. In a gas bulb resulting electric field accelerates the electrons and ions in the gas discharge and they collide, transferring part of the kinetic energy into internal energy of the gas ions. The electrons in the gas ions are “lifted” in an excited state. They reside in this state for relatively short time and then they relax to a lower state by emitting a photon, which we utilize for illumination. Such impact ionization is unwanted in OLEDs, besides it is hard to achieve in a controllable manner.
In an OLED the electrodes are typically chosen such that the electrons in the anode are energetically close to the energy level where most of the electrons reside, i.e. the highest occupied molecular orbital (HOMO) or valence band. The cathode is typically chosen so that the electrons in it are energetically close to the first unoccupied energy level above the HOMO, i.e. the lowest unoccupied molecular orbital (LUMO) or conduction band.
If positive potential is applied to the anode and negative to the cathode in an OLED, the electrons from the cathode are injected in the polymer layer in its LUMO and electrons from the HOMO of the polymer are extracted into the anode. Effectively a positive charge, or a hole in the sea of electrons, is left at the position of the extracted electron, which is why the process is described as injection of holes from the anode into the polymer. The injected hole and electron drift into the polymer layer and when they meet the electron can recombine with the hole, or the system relaxes to the ground state, by emission of energy in the form of photons.
A critical step in the synthesis of OLEDs according to present methods is the deposition of the cathode. Typically the cathode is a metal layer, sometimes with a very thin buffer layer of elements from the first or second period of the Periodic Table. Deposition is usually done in vacuum systems by thermal evaporation or sputtering.
In other processes for creation of OLEDs, a conducting paste based on silver, nickel, carbon, and antimony-doped tin oxide particles is embedded in polymer matrix, which makes it suitable for screen printing of OLEDs or use in formulating inks for jet printing of OLEDs. Another approach is proposed by Lochun, et al. “Manufacturing flexible light-emitting polymer displays with conductive lithographic film technology.” Smart Materials & Structures 10, 650-656 (2001). Lochun's lithographic approach is applied only to the anode, and the cathode is processed by traditional deposition using vacuum evaporation. Ocypa, et al. “Electroless silver deposition on polypyrrole and poly(3,4-ethylenedioxythiophene): The reaction/diffusion balance” J. Electroanalytical Chem. 596, 157-168 (2006), reported the electroless deposition of silver on top of polypyrrole and poly(3,4-ethylenedioxythiophene); however, no further device application is demonstrated, and there is no apparent motivation to provide a device. Krebs et al. “A complete process for production of flexible large area polymer solar cells entirely using screen printing-First public demonstration” Solar Energy Materials and Solar Cells 93, 422-441 (2009) and Zeng et al. “Polymer light-emitting diodes with cathodes printed from conducting Ag paste” Advanced Materials 19, 810 (2007), used screen printing of commercial silver paste to deposit silver films with significant thickness of several micrometers. The lamination process is another strategy for avoiding direct deposition of the metal film on top of the light emitting polymer.